1. Field of Invention
This invention belongs to the area of flat panel display and more specifically to the backlight technology for Liquid Crystal Display (LCD).
LCD does not emit light and hence requires a backlight for its function as a visual display. Backlights based on Cold Cathode Fluorescent lamps (CCFLs) have been employed for backlighting LCDs. Recently Light Emitting Diodes (LEDs) have been employed as light sources for backlighting LCDs. White LEDs are predominantly used in cell phones and digital cameras and recently in lap top computers for backlighting LCDs. Red, Blue and Green LEDs are used in backlighting LCD TV. LCD requires a flat or sheet source of light at its rear side. As LEDs are point sources of light, there is a need to convert the point source of light in to a sheet source of light. This conversion is done through two configurations namely, edge-lit configuration and direct-lit configuration. In an edge-lit configuration, the LEDs are assembled at the edges of a rectangular light guide, usually a polycarbonate slab, and the light from LEDs undergo total internal reflections at the inner surface of light guide and finally get extracted through top surface of light guide facing the back surface of LCD. In the direct-lit configuration, LEDs are assembled inside a box and a diffuser sheet is assembled over the LEDs. Light from LEDs get mixed inside the box and further mixed in the diffuser sheet and finally emanates from the diffuser sheet towards LCD.
Thus a sheet source of light, usually white light, is generated and provided as backlight for LCD. The white light from LEDs contains all three primary colors and the colors are further filtered by color filters incorporated inside LCD for generating good quality of color pictures. It is well known that the color filters absorb 70-80% of incoming light and hence the LCD becomes optically less efficient. This remains a challenge.
2. Description of Prior Art
Prior art deals with the elimination of color filters inside LCD and provision of three primary colors of good purity that requires no filtering by the color filters. For example in one prior art (U.S. Pat. No. 4,978,888), Anandan et.al describes the use of flat fluorescent lamp incorporating red, blue and green color phosphors in the form of pixels, for backlighting color filter-less LCD. Red, blue and green color light from the pixilated surface of the flat fluorescent lamp provides individual primary colors of light to the pixels of LCD that does not incorporate color filters. Hence the use of the term ‘color pixel backlighting’. The drawback in this prior art is the high ignition voltage in the range of thousands of volts and running voltage in the range of hundreds of volts in addition to thick and massive glass plates employed for the flat light source.
In another prior art (M. J. J. Jack et.al—“Color Separating backlight for improved LCD efficiency”, pp. 175-178, Proceedings of 27th International Display Research Conference, 18-20 Sep. 2007, Moscow, Russia; Yoichi Taira et.al, “Low power LCD using a novel optical system”, SID-02 Digest, pp. 1313-1315, 2002; Jyrki Kimmel et.al, “Diffractive backlight grating array for mobile displays”, Proceedings of IDRC 2007, pp. 171-174, Moscow, September 2007) the authors describe the use of diffraction grating over the light guide to separate the three primary colors from the white light and employ a sheet of micro-lenses. The diffraction grating-separated three primary colors go through the LCD that has no color filters. The drawback in this prior art is the ‘color cross-talk’. That is, light of one color passes through the unintended pixel of LCD where a different colored light is incident. It is clear from the prior art that providing a simple solution for ‘color pixel backlighting’ for a color filter-less LCD is a challenge.
In yet another prior art (U.S. Pat. No. 7,036,946 B1) Donald E. Mosier describes LCD backlight employing UV LEDs. In this invention, UV light from LEDs is coupled to a light guide and the UV light is made to fall on phosphor coated on the surface of light guide to emit visible light towards the LCD. This invention is completely devoid of the concept of ‘color pixel backlighting’ and does not deal with LCD without color filters.
A lower wavelength of light when incident on materials like phosphor results in the generation of higher wavelength of light. This is a basic science and known phenomena that is employed in fluorescent lamp for the past 40 years. Another example of lower wavelength light exciting a phosphor is the blue light from LED exciting yellow phosphor coated on the LED chip. This is commonly found in all the white LEDs employed in cell phone backlighting. This basic science is well known. What is not obvious is the innovative ways in which this basic science is exploited to invent new devices. U.S. Pat. No. 7,036,946 B1 by Donald E. Mosier exploits this basic science in inventing a light guide using UV light from LEDs. This invention does not contemplate about the backlighting of color filter-less LCD through the concept of ‘color pixel backlighting’. By reading U.S. Pat. No. 7,036,946 B1, it will not be obvious to a person of ordinary skill to envision the concept of color pixel backlighting and exploit the basic science of lower wavelength light exciting a phosphor to invent a light guide that can backlight a color filter-less LCD. The present invention described in the subsequent paragraphs will not fall under the category of ‘obviousness’.
In a research paper (N. Ogawa et.al—“Filed sequential color, LCD using organic EL backlighting”—SID Digest of Technical Papers SID International Symposium 1999), N. Ogawa et.al describe organic based electroluminescent backlight that is backlighting an LCD without color filters. This art is based on the field sequential operation of LCD wherein each frame of LCD driving consists of only one color. For example the backlight will provide a sheet of red light to the LCD 60 times per second and the LCD will select red pixels during this period. The next will be green sheet of light from backlight for 60 times a second followed by blue sheet of light. In other words the backlight will provide continuous sheet of light with a frame rate of 60 Hz for each color. This takes place sequentially for each color during which time the desired color pixels will be selected on LCD to generate color images. In this art, there is no ‘color pixel backlighting’ that takes place continuously. This is totally a different mode of operation and the sequential flashing of color sheet of light on LCD results in Psycho-physical effect that makes display exhibit ‘color break-up’, a defect that is still to be solved to make any commercial product.
In all the foregoing inventions, it is clear that either the concept of ‘color pixel backlighting’ is absent or, if present, it has problems for practical implementation and thus there is no simple solution for the challenge posed by ‘color pixel backlighting’ of color filter-less LCD. The present invention provides a simple solution to the challenge posed by ‘color pixel backlighting’.